Human NK receptors: from the molecules to the therapy of high risk leukemias.

Natural killer cells are important players of the innate immunity. In humans, they express HLA-class I-specific inhibitory receptors including the allotypic-specific KIR and various activating receptors. In most instances, in an autologous setting NK cells do not kill self cells. In contrast, in an allogeneic setting as the haploidentical hematopoietic stem cell transplantation to cure high risk leukemias, donor-derived NK cells may express inhibitory KIR that are not engaged by the HLA-class I alleles (KIR ligands) expressed by recipient cells. Such "alloreactive" NK cells may be responsible for the eradication of leukemia blasts escaping the preparative regimen, residual host dendritic cells and T lymphocytes, thus preventing leukemia relapse, GvHD and graft rejection, respectively. These NK-mediated effects result in a sharp improvement of the estimated 5 years survival.